This research focuses on oxidative modification of proteins. The resulting covalent modifications have been implicated in important physiologic and pathologic processes. During this year, we continued our emphasis on the physiological functions of oxidative modification of proteins, on the functions of oxidatively modified proteins, and on therapeutic intervention in the processes. Iron-responsive protein-2 is a 100 kD protein which binds to mRNA targets, altering both translation efficiency and RNA stability. We previously showed that a 7 kD region of the protein binds iron with high affinity. We now show that it functions as a high-affinity heme-binding domain and thus as a heme sensor. The heme-bound peptide mediates an oxygen-dependent beta-elimination of a cysteine residue. We showed previously that the mouse model of the neurodegenerative disease ataxia- telangiectasia, ATM, is characterized by oxidative stress and oxidative modification of macromolecules. This year we published a therapeutic trial in the mice which demonstrated that treatment with a catalytic antioxidant, EUK-189, corrected the neurobehavioral abnormality in the mice. Carbonic anhydrase 3 (Car3) is a cytosolic protein which is particularly abundant in skeletal muscle, adipocytes, and liver. The specific activity of this isozyme is quite low, suggesting that its physiological function is not that of hydrating carbon dioxide. To understand the cellular roles of carbonic anhydrase 3, we inactivated the Car3 gene. Mice lacking carbonic anhydrase 3 were viable, fertile, and had normal lifespan. Carbonic anhydrase 3 has also been implicated in the response to oxidative stress. We found that mice lacking the protein had the same response to a hyperoxic challenge as their wild-type siblings. No anatomic alterations were noted in the mice lacking carbonic anhydrase 3. They had normal amounts and distribution of fat, despite the fact that carbonic anhydrase 3 constitutes about 30% of the soluble protein in adipocytes. We conclude that carbonic anhydrase 3 is dispensable to mice living under standard laboratory husbandry conditions.